Ring-C-14-labelled 4(3'-, 6'-, dimethyl-3'-,heptyl)-phenol (NP), an isomer of paranonyl phenol was synthesized for use in aquatic toxicity and metabolism studies. A very efficient method involving alkylation of ring-C-14-labeled anisole and tertiary alkyl bromide (3-bromo-3,6-dimethyl heptane) with AlCl3 followed by cleavage of the resultant ether with BBr3 was used giving a 24% yield ring labelled C-14-NP (specific activity: 16.205 MBq/mg and purity > 95% by HPLC) after TLC separation.